An EUV radiation generating apparatus having a beam guiding device for guiding a laser beam to a target position is known from US 2011/0140008 A1. The beam guiding device described therein is used to guide laser radiation that has been produced and amplified in a driver laser system. Generally a CO2 laser is used as the driver laser, because it allows a high level of conversion efficiency between the input power of the driver laser and the output power of the produced EUV radiation with specific target materials, for example, tin. The beam guiding device guides the laser beam to a focusing element or a focusing device which is used to focus the laser beam at the target position. There is provided at the target position a target material that changes into a plasma state during irradiation with the laser beam and emits EUV radiation.
During irradiation with the laser beam, a portion of the target material (for example, tin) is typically vaporized and may be deposited on the optical surfaces of optical elements that are arranged in the vicinity of the target position. Laser beams having a relatively large wavelength of, for example, approximately 10.6 μm (e.g., when using a CO2 laser), will generally be reflected by optical elements that have a comparatively rough optical surface caused by such tin deposits. However, in optical elements that serve to reflect EUV radiation (with a much smaller wavelength) produced at the target position, this is not generally the case, because deposits of contaminating materials on those optical elements typically result in a significant reduction in the reflectivity for the EUV radiation used. Thus, optical elements for use with EVU radiation that are arranged in subassemblies in a vacuum chamber or arranged downstream thereof, for example, an illumination system and optical elements arranged in a projection system, should be protected from such contaminating materials, in particular deposits of the target material.